1. Field of the Invention
The present invention relates to an image pickup apparatus having an image pickup device such as an inter-line type CCD (referred to as "IL-CCD" hereinafter), an X-Y address-type MOS image sensor, a non-destructive image pickup device (referred to as "NDI" hereinafter), and so forth.
2. Description of the Prior Art
Image-pickup apparatus having a CCD, such as solid-state TV cameras, have been known. In this type of image pickup apparatus, the CCD reads the signals contained in each horizontal line of picture elements constituting an image in an interlaced manner and the thus read signals are delayed by a time corresponding to the period of scanning of one or two such horizontal lines. The delayed signals are utilized in the formation of a luminance signal and color signals, through a vertical correlation processing which makes use of the vertical correlation between the horizontal lines constituting an image.
In another type of image pickup apparatus incorporating a device known as a MOS image sensor, the luminance signals and color signals are obtained through concurrent reading of a plurality of horizontal lines, without the need for any delay line.
These known image pickup apparatus suffer from a disadvantage in that the quality of the reproduced image is impaired by moire and/or false signals due to the fact that the vertical correlative distance is inevitably increased as a result of the interlace scanning In addition, the image pickup apparatus incorporating IL-CCD requires a large-scale peripheral circuit due to the use of numerous delay lines, with the result that the cost of the apparatus is raised undesirably.
It has been proposed to operate the IL-CCD type image pickup apparatus in a specific mode known as "field accumulation mode", in order to reduce the frame after-image. In this case, however, the color reproducibility is impaired due to the use of a modulation-type image pickup system. In addition, the resolution is deteriorated in the vertical direction because the reading output is obtained as an algebraic sum of the charges of two horizontal lines. Furthermore, since the color difference signals are obtained only in accordance with the line sequence, false signals are inevitably formed during summing of the color difference signals due to the differences in the low-band component of the color difference signal and the low-band component of the luminance signal.
These problems will be explained with reference to FIGS. 1 and 2. Referring first to FIG. 1, which shows a typical example of a conventional, striped color separation filter, horizontal scanning lines for constituting an odd number field are represented by o1, o2 and o3, while horizontal scanning lines for forming an even number field are represented by e1, e2 and e3. During the scanning along each horizontal scanning line i, signals Ri, Gi and Bi are read, and from the thus read signals, a luminance signal Y.sub.i and color difference signals R.sub.i -Y.sub.i and B.sub.i -Y.sub.i are formed, in accordance with, for example, the following formula: EQU Y.sub.i =0.3R.sub.i +0.59G.sub.i +0.11B.sub.i
These color difference signals R.sub.i -Y.sub.i and B.sub.i -Y.sub.i are transmitted and recorded in accordance with line sequential. In this type of filter, however, the resolution is comparatively low in the horizontal direction due to the fact that the pitch or interval of the filter elements of the same color in the horizontal direction is comparatively large, although the generation of false signals is suppressed appreciably. In order to improve the resolution in the horizontal direction, a filter has been proposed in which, as shown in FIG. 2, filter elements of the same color appear at every second picture element in the horizontal direction. The luminance signals and color difference signals obtained through scanning of respective horizontal scanning lines for constituting odd number fields are set on Table 1.
TABLE 1 ______________________________________ Color- Scan- Dif. ning Lumin- line Recomposed Signal line ance seq. Summing R B ______________________________________ o2 Y.sub.3 R.sub.3 - Y.sub.3 B.sub.1 - Y.sub.1 R.sub.3 B.sub.1 + (Y.sub.3 -Y.sub.1) o3 Y.sub.5 B.sub.5 - Y.sub.5 R.sub.3 - Y.sub.3 R.sub.3 + (Y.sub.5 -Y.sub.3) B.sub.5 o4 Y.sub.7 R.sub.7 - Y.sub.7 B.sub.5 - Y.sub.5 R.sub.7 B.sub.5 + (Y.sub.7 -Y.sub.5) ______________________________________
In the operation of an image pickup apparatus incorporating a color separation filter of the type shown in FIG. 2, the scanning output obtained through scanning each horizontal line lacks a color difference signal corresponding to the color omitted from a particular horizontal line of color separation filter elements, and a color difference signal obtained from the preceding horizontal scanning line is used in place of the missing signal after suitable vertical correlative processing. In this case, therefore, the signals R and B (G being omitted) recomposed by the summing of the luminance signal and the color difference signal obtained from the preceding horizontal scanning line inevitably involves a false signal attributable to the difference in the luminance between these two horizontal scanning lines, as is the case with the signal B obtained from the horizontal scanning line o2 which includes a term (Y.sub.3 -Y.sub.1) corresponding to the difference in the luminance between the horizontal scanning lines o1 and o2.
This problem is attributable to the fact that, due to the interlace scanning method employed in the television system, a vertical correlative distance corresponding to two picture elements in the vertical direction is formed between the color difference signal and the luminance signal which are to be summed for recomposing the color signal.
Another problem encountered with the known solid-state image pickup device is that the sensitivity of the device is limited by the primary color signal, due to the fact that the picture elements for obtaining complementary colors and the picture elements for obtaining primary colors have an equal light-receiving area. In general, the image pickup device exhibits a sensitivity to a primary color transmitted through a primary color filter which is about 3 to 4 times as high as the sensitivity to complementry colors which have passed the total-color transmitting filter. Therefore, if the sensitivity of the image pickup device is optimized for the complementary colors, the device becomes too sensitive to the primary colors. It is therefore not practical to increase the sensitivity unlimitedly.
Still another problem of the known solid-state image pickup device pertains to a difference between the S/N value of the luminance signal and the S/N value of the color signal. The S/N value of the color signal is smaller than that of the luminance signal even when the difference in the bandwidth between these signals is taken into consideration. For instance, assuming here that the luminance signal and the color signal have bandwidths of 4 MHz and 1 MHz, respectively, taking into consideration the bandwidth difference raises the S/N value of the color signal by about 6 dB as compared with the S/N value of the luminance signal, but the S/N value of the color signal is still lower than the S/N value of the luminance signal by about 3 to 5 dB.
A further problem encountered by the known image pickup device pertains to a fluctuation in the dark current in the sensor attributable to a difference in the sensitivity between the picture elements for primary colors and the picture elements for complementary colors. In particular, the fluctuation of the dark current existing in the narrow color signal band cannot be suppressed by the reduction in the difference in sensitivity attained by the restriction of the bandwidth.
A further problem is that the dynamic range of the sensor is undesirably limited due to the difference in the S/N value between the color signal and the luminance signal.
A further problem is that difficulty is encountered in the assembly of image pickup elements, particularly in a multi-plate image pickup apparatus, with regard to attaining sufficiently high precision of alignment of the sensor elements.